Method of Melting A Mixture of Scrap Metal Using Scrap Rubber

ABSTRACT

A process for melting scrap metal for producing steel which includes the steps of combining a quantity of scrap metal and scrap rubber in an electric arc furnace. Energy is then applied to the quantity of steel and scrap rubber in the furnace to start the combustion of the scrap rubber to add additional heat for melting the scrap metal containing steel.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of Ser. No. 12/203,742 filed Sep. 3,2008, now U.S. Pat. No. 7,674,318, which is a continuation of Ser. No.09/974,199 filed Oct. 10, 2001, which is a continuation-in-partapplication of Ser. No. 09/311,401 filed May 13, 1999.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to the manufacturing of steel in afurnace and more particularly to the manufacturing of steel from scrapmetal in an electric arc furnace using scrap automotive tires as anauxiliary heat supplying source.

2. Description of the Prior Art

Scrap automotive tires present an environmental problem and recycling ispractically nonexistent. Tires do not degrade in landfills and whenstockpiled, create a major fire hazard that is impossible to extinguishonce ignited. Since they have about the same heating value as coke,15,000 BTU's per pound, a tire weighing about 20 pounds hasapproximately 300,000 BTU's. The scrap automotive tires are so plentifulthat they have a near zero cost. At the present time, scrap automotivetires are being used as fuel or auxiliary fuel in a variety ofoperations such as cement kilns, coal fired generators and otherapplications wherein a controlled firing rate is used. In suchinstances, it is often necessary to shred the scrap automotive tiresprior to using in a furnace. Also, when the scrap automotive tirescontain steel belts, it is often necessary to remove the steel belts. Insome instances whole rubber tires have been used but such use requiredequipment changes that reduced the cost advantages. In the manufacturingof steel from scrap metal, some of the steel mills add coke or coal tothe scrap metal that is melted in a furnace such as an electric arcfurnace.

An Electric Arc Furnace (EAF) is not an ordinary furnace in any sense ofthe word. It is a vessel for melting steel and providing metallurgicalprocessing. It is believed that the EAF is the best choice for use ofthe tires, or as it is often referred to, tire derived fuel (“TDF”), asit can overcome all of the specific problems associated with the steelbelts and beads when using TDF.

Tires are sometimes used as a fuel or supplemental fuel in thegeneration of electric power. Because of the problems created by thesteel in the TDF they are not widely used or accepted as a fuel byelectric utilities. Whether or not tires are used, the electric powerhas to be converted to higher voltages, transmitted to the steel milland then converted back into heat by the electric arc. By using tiresdirectly as energy in the steel furnace all of the losses intransmission and conversion are eliminated.

Because of the problems created by the steel belts and beads, the amountof tires now being used is not large enough to consume the amount oftires generated each year as scrap. TDF is the largest use for scraptire disposal but the amount of tires scraped each year far exceeds thenumber that can be consumed by all of the various outlets combined. Alarge portion are shredded and buried in landfills. Only the steelindustry, as a very large consumer of energy, has the capability toconsume tires in an amount approaching that of the rate of disposal.Therefore, it should be noted that not only does the burning of tiresfor fuel increase the efficiency of the EAF, it also provides a uniquesolution for disposing of tires in an environmentally sound method. Itprovides an alternative to wasting and squandering valuable energy at atime when energy is becoming more scarce and costly. Furthermore, evenwhen the steel industry decides to reuse the steel belts from the tires,the tires are stripped or shredded to remove the rubber to access thesteel belts. Much of the rubber is then discarded, which provides only alimited solution to the problem of the waste rubber. There is thereforea need for a process which will utilize both the steel and the rubberfound in most tires, and which does so in an efficient and productivemanner.

In the EAF, coal or coke is added to the scrap charge as a source ofchemical energy but also as an additional source of carbon for the steelbeing manufactured. The addition of rubber reduces or eliminates theneed for coal or coke as a carbon source for steel chemistryrequirements.

Therefore, an object of the present invention is to provide an improvedmethod of melting scrap metal using scrap rubber.

Another object of the present invention is to provide an improved methodof melting scrap metal using scrap rubber which includes the steps ofcombining a quantity comprising scrap metal containing steel and atleast about 0.25 percent by weight of scrap rubber, forming a bundle ofthe combined scrap metal and scrap rubber, placing the bundle in anelectric arc furnace and applying energy to the quantity in the furnaceto start the combustion of the scrap rubber to add additional heat formelting the scrap metal containing steel.

Another object of the present invention is to provide an improved methodof melting scrap metal using scrap rubber which will use various typesof scrap rubber, including chopped, shredded and even whole tires baledand unbaled, both with the steel belt included and without.

Another object of the present invention is to provide an improved methodof melting scrap metal using scrap rubber in which the pollution emittedfrom the steel plant is greatly reduced, because, specifically, whenscrap rubber tires are added to the melting steel, the carbon monoxideemissions that normally occur from the arc furnace are greatly reduced.

Another object of the present invention is to provide an improved methodof melting scrap metal using scrap rubber which includes a separateburning container for the tires which is in fluid transmissionconnection with the furnace such that the heat produced by the burningtires in the container is transferable to the furnace without addingimpurities caused by the introduction of steel belts from the tires.

Finally, an object of the present invention is to provide an improvedmethod of melting scrap metal using scrap rubber which is safe,efficient and environmentally sound in use.

SUMMARY OF THE INVENTION

This invention relates to a method for melting steel using scrap metaland at least about 0.25 percent by weight of scrap rubber, such as scrapautomotive tires, wherein scrap metal and scrap rubber tires aredeposited in a steel melting furnace, such as an electric arc furnace,and the scrap rubber tires or pieces thereof are combusted with air oroxygen to provide an auxiliary source of heat to melt the scrap metal.

In the preferred embodiment of the invention, an electric arc furnace isused. In the preferred method, a quantity of scrap metal with or withoutrubber is deposited in the electric arc furnace and heat is appliedthereto to form a molten pool of metal. The scrap metal is theconventional scrap metal used to make steel. A quantity of scrap rubber,preferably scrap rubber tires, in an amount of at least 0.25 percent byweight, is then loaded into a bottom opening bucket and another quantityof scrap metal is loaded into the bucket on top of the scrap rubbertires. The bottom of the bucket is then opened and the scrap rubbertires fall into the electric arc furnace followed by the scrap metal. Ifdesired, some of the scrap rubber tires could be included in the firstquantity of scrap metal melted in the furnace. In addition to theelectrodes, the furnace may have oxygen/air blow pipes or oxygen/naturalgas burners to assist in the melting of the scrap metal and in thecombustion of the scrap rubber tires. The scrap rubber tires ignite andare combusted to add auxiliary heat to the furnace. Once the scraprubber tires are ignited, the natural gas is turned off and the oxygenis available for the combustion of the scrap rubber tires. Theelectrodes in the electric arc furnace continue to operate and functionto control the temperature in the furnace. Additional charges of scrapmetal or scrap metal and scrap rubber are subsequently added into thefurnace until its capacity has been reached. Of course, the temperaturein the furnace would preferably be the temperature normally used in themaking of steel from scrap metal which is about 2950 degrees Fahrenheit.

In another preferred embodiment of the invention, a container for thepyrolysis of the scrap whole, or cut, or shredded and de-wired rubbertires is located adjacent to an electric arc furnace. Hot exhaust gasesfrom the electric arc furnace are fed into a jacket surrounding thecontainer to heat the whole, cut or shredded and de-wired rubber tiresand convert them to combustible liquids and gases. Suitable controlmeans are provided to feed the resulting combustibles into the electricarc furnace to function as an auxiliary source of heat during thecombustion thereof.

In still another embodiment of the present invention, the scrap metaland scrap rubber are combined to form a scrap metal and rubber bundle,with the scrap rubber intermixed with the scrap metal. The metal acts asa flame retardant and also as a heat sink, thereby preventing rapid anduncontrolled burning of the scrap rubber. Further, the bundles may bepicked up by a scrap magnet much as is done with the standard scrapmetal bundles found in the industry today. The scrap metal/scrap rubberbundle thus provides a controlled rate of burning while also permittingrelatively easy handling of the scrap rubber and scrap metal.

In yet another embodiment, the shredded rubber is added and intermixedwith the shredded steel scrap to form a generally homogenous mixture.This mixture can then be readily handled by a magnet for easy insertionof the mixture into the EAF. It is thus seen that the present inventionprovides a substantial improvement over the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of apparatus for use in one preferredembodiment of the invention.

FIG. 2 is a schematic illustration of apparatus for use in anotherpreferred embodiment of the invention.

FIG. 3 is a schematic illustration of apparatus for use in yet anotherpreferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is shown best in FIG. 1, which illustrates anelectric arc furnace 2 of the type generally used in a manufacturingoperation to convert scrap metal into steel and is provided with apouring spout 4, bottom tap (not shown) or the like. The roof 6 with theelectrodes 8 has been raised and swung aside. A charging bucket 10having bottom doors 12 has been positioned over the electric arc furnace2 and is supported by a pair of hooks 14 (only one shown) engagingoutward projecting integral studs 16 (only one shown). The hooks 14 arepart of a conventional crane (not shown) which moves the charging bucket10 to the position over the electric arc furnace. Oxygen/natural gasburners 18 extend through the sidewall or roof of the electric arcfurnace and are used to assist in the melting of the scrap metal and theignition of scrap rubber tires as described below. Once the scrap rubbertires have been ignited, the natural gas is turned off and the oxygen isavailable for the combustion of the scrap rubber tires.

It is to be understood that although the present invention is describedas using a charging bucket to add the scrap metal and rubber, moremodern plants may add the scrap metal to the arc furnace via conveyorsor shafts which permit the furnace to remain closed during the adding ofscrap metal. The scrap rubber would be added to the conveyor or shaft inthe desired amounts and at the proper time and location to enable themethod of the present invention to be employed.

In a preferred process of this invention, a first quantity of scrapmetal (not shown) of the type conventionally used in the manufacture ofsteel from scrap metal has been loaded into the charging bucket 10 andmoved to the position illustrated in FIG. 1. A quantity of scrap rubbertires is also placed in the charging bucket 10 and the scrap metal andrubber is then dropped into the electric arc furnace 2. The chargingbucket 10 is then removed and the roof 6 with the electrodes 8 mountedthereon is moved to an operating position in the electric arc furnace 2.An electric current is supplied to the electrodes 8 and theoxygen/natural gas burners 18 are ignited to generate heat to melt thescrap metal. In one example of the process using an electric arc furnacehaving a capacity of 100,000 pounds, the first quantity of scrap metalcomprised 30,000 pounds and the first quantity of scrap rubber comprisedapproximately 1,000 pounds.

During the melting of the first quantity of scrap metal, the chargingbucket 10 is loaded with a second quantity of materials. The secondquantity of materials comprises a quantity of scrap rubber tires whichare first loaded into the charging bucket and a second quantity of scrapmetal that is loaded into the charging bucket on top of the scrap rubbertires. It is important to note that various forms of scrap rubber tirescan be used with the method of the present invention, including chopped,shredded, whole tires and even tire bales, both with the scrap steelincluded and without. It has been found that depending upon the desiredrate of burn (infusion of energy from the burning tires), the size andamount of the scrap tire pieces can be modified and changed, with thequickest heat addition coming when shredded tires are used and thelongest lasting heat addition coming when whole tires are used. The heatinflux provided by the tires can be calculated such that a practitionerof the present invention would be able to generally control the amountand timing of heat influx, which is critical in the operation of the EAFand in the creation of certain alloys and other metal combinations.

After the charging bucket is filled with the scrap rubber tires and thescrap metal, the roof 6 with the electrodes 8 is raised and swung aside.The charging bucket 10 is moved to a position over the electric arcfurnace 2 and the bottom doors 12 are opened to drop the scrap rubbertires followed by the additional scrap metal into the electric arefurnace 2. The charging bucket is moved out of the way and the roof 6with the electrodes 8 is moved back onto the electric arc furnace 2. Theheat of the molten scrap metal in the electric arc furnace 2 and theheat generated by the electrodes 8 together with the oxygen/natural gasburners 18 in the furnace function to ignite the scrap rubber tires andtheir combustion with air or oxygen functions to produce auxiliary heatto heat the additional scrap metal as the heat moves through theadditional scrap metal. The electrodes 8 continue to operate to controlthe temperature of the molten material and to assist in the melting ofthe additional scrap metal but the power supplied to the electrodes 8 isreduced as a result of the heat generated by the combustion of the scraprubber tires. As stated above, once the scrap rubber tires have beenignited, the natural gas is turned off and the oxygen is available forthe combustion of the scrap rubber tires. If the electric arc furnace 2is not equipped with oxygen/natural gas burners, a conventional lance orblow pipe may be used to provide the oxygen for the combustion of thescrap rubber tires, in addition to other methods such as the admissionof air into the furnace through doors, dampers or other such openings.In accordance with the example described above, the second quantity ofmaterials comprises about 600 pounds of scrap rubber tires and about30,000 pounds of additional scrap metal. The temperature in the electricarc furnace is the conventional temperature used to melt scrap metalwhich is about 2950 degrees Fahrenheit. If desired, a quantity of thescrap rubber tires, such as about 100 to 1000 pounds, can be includedwith the first quantity of scrap metal.

During the melting of the second quantity of scrap metal, a secondquantity of materials comprising a second quantity of scrap rubber tiresand a third quantity of scrap metal is loaded into the charging bucket10. When the second quantity of scrap metal has been melted, the roof 6with the electrodes 8 and the charging bucket 10 are moved to drop thethird quantity of scrap metal into the electric arc furnace 2, thecharging bucket 10 is moved away and the roof 6 with the electrodes 8are moved back into the operating position. In accordance with theexample described above, the second quantity of scrap rubber tires isabout 500 pounds and the third quantity of scrap metal is about 20,000pounds.

The process is then repeated to add a third quantity of materialscomprising a third quantity of scrap rubber tires and a fourth quantityof scrap metal necessary to reach the capacity of the electric arcfurnace 2. If the scrap rubber tires have steel belts, then the steel inthe steel belts becomes part of the molten steel in the electric arcfurnace 2. The third quantity of scrap rubber tires is in an amount ofabout 500 pounds.

In the example described above, the second quantity of materials wasadded to the electric arc furnace 2 about 15 minutes after the currentwas supplied to the electrodes to commence the melting of the firstquantity of scrap metal. The third and fourth quantities of materialsare added in successive intervals spaced about 15 minutes apart. Afterabout another 15 minutes, normal refining processes are performed on themolten metal to obtain desired characteristics. It is to be understoodthat the foregoing is only an example and that other quantities andratios of scrap metal and scrap tires and other sizes of furnaces may beused.

While it may be preferable in general melting processes to use wholescrap rubber tires so as to control the combustion thereof, if shreddedscrap rubber tire were to be used, the combustion would occur veryrapidly and generate an amount of heat far greater than that produced bythe combustion of the whole scrap rubber tires. A variation foradditional control of combustion is shown in FIG. 3 in which scrap metaland scrap tire bundles 50 are dropped into a furnace 40 through anoptimally located opening 46, the furnace 40 already containing aquantity of molten scrap metal and unmelted scrap 48. Two oxy/fuelburners 42 a and 42 b extend into the furnace 40 for igniting the tiresand providing combustion oxygen. As the scrap tires burn, energy isadded to the mass of scrap metal 48 and the scrap metal in the bundlesand the remaining unmelted scrap metal is melted at a faster and moreefficient rate than that ordinarily obtained. Molten slag floats to thetop of the molten metal and the molten scrap metal is then poured out ofthe furnace 40 through pouring spout 44. This embodiment presentssignificant advantages over the prior art and even is superior to merelycombining the scrap rubber in the metal. With the scrap metal and scraprubber being combined to form a scrap metal and rubber bundle, i.e., thescrap rubber intermixed with the scrap metal, the metal acts as aflame-spread reducer and also as a heat sink, thereby preventing rapidand uncontrolled burning of the scrap rubber. Further, the bundles maybe picked up by a scrap magnet much as is done with the standard scrapmetal bundles found in the industry today. The scrap metal/scrap rubberbundle thus provides a controlled rate of burning while also permittingrelatively easy handling of the scrap rubber and scrap metal.

An additional important feature of the present invention is that thepollution emitted from the steel plant is greatly reduced when scraprubber tires are added to the melting steel. When tires are added to thescrap metal the carbon monoxide emissions that normally occur from thearc furnace are greatly reduced. The tires act as a catalyst for carbonmonoxide to carbon dioxide conversion. This also results in a muchgreater release of usable energy inside the furnace converting theenergy therein where it has the most benefit for the melting of thesteel. When conventional coal or coke is used, a lot of the availablechemical energy is lost because the carbon in the fuel does notcompletely convert to carbon dioxide during the burning process.Numerous studies have been done looking into ways to increase theconversion rate and thus increase the efficiency of the melting process,including such methods as additional oxygen injection and other suchtechniques. The scrap rubber tires used in the method of the presentinvention provide at least a partial solution to this problem even ifused only as a supplement to the coal or coke just for this purpose.More than half of the available energy is lost if the carbon is notallowed to react all the way to CO₂. Often the carbon monoxide convertsin the duct system outside of the furnace resulting in wasted energy,duct damage or even explosion, as it is a combustible gas. It has beenfound that the addition of the scrap tires as taught in the presentinvention will at least provide a partial solution to these problems.The inclusion of scrap tires in the melting process also has been shownto reduce NOx and SO₂ emissions when properly combusted in the EAF. Thisis another very important environmental consideration in steel milloperations and environmental operating permit compliance.

The use of TDF in the EAF also reduces the air pollution from the EAFprocess. TDF contains less carbon and therefore produces less carbondioxide than coal or coke whether it is burned at the EAF or back at theelectric utility. TDF contains twice as much hydrogen and burns hotter,i.e., it contains more fuel value. Additionally the hydrogen acts as acatalyst and reduces CO emissions, a very important and environmentallysignificant goal in the steel industry. The shredded tires can also bemixed with coal or coke and used to reduce CO emissions with a resultbeing a greatly increased energy release from CO to CO₂ conversion.

An apparatus for practicing another preferred embodiment of theinvention is schematically illustrated in FIG. 2. Apparatuses similar tothat shown in FIG. 1 have been given the same reference numerals. InFIG. 2, a container 20 is mounted at a fixed location by conventionalmounting means (not shown) so that it is close to the electric arcfurnace 2 and has a removable control damper/cover 22 which is used toadjust the amount of combustible air available to the tires 100 whichare to be burned within the container 20 thus regulating the rate ofheat in the process. At least one oxy/gas burner 18 is connected to thecontainer 20 to ignite the tires 100 within the container 20, theoxy/gas burner 18 operating in a manner similar to that describedpreviously in connection with the first embodiment. A pipe 30 extendsbetween and is in fluid communication with the container 20 and theelectric arc furnace 2. A fume collecting system duct 24 is in fluidcommunication with the interior of the electric arc furnace 2 so thatthe heat generated by the burning of the tires 100 is drawn into theelectric arc furnace 2, the heat passing through the fume collectingsystem duct 24. A molten metal conduit 4 is in fluid communication withthe electric arc furnace 2 for releasing the molten metal from theelectric arc furnace 2. As the fume collecting system duct 24 draws thesuperheated air from the container 20 into the electric arc furnace 2around the metal being melted in the electric arc furnace 2, the heatgenerated by the burning tires adds to the heat generated by the oxy/gasburners in the electric arc furnace 2 thus decreasing the time andenergy required to melt the steel and thus lowering the cost of steelproduction.

In practice, a quantity of whole, cut or shredded and de-wired rubbertires (not shown) is deposited into the container 20 and the oxy/gasburners 18 are used to ignite the whole, cut or shredded and de-wiredrubber tires 100 to form combustible liquids and gases. The combustibleliquids and gases flow through the pipe 30 thus adding their energyproduced by combustion to the steel being melted in the electric arcfurnace 2.

Still another embodiment of the present invention would include asomewhat modified container 20 as described in connection with FIG. 2.One of the problems with using tire derived fuel is the steel belts andbeads that they contain. The steel generally does not belong in acombustion chamber as typically found in a coal or gas fired boiler orsimilar fuel burning system.

The tire-burning container 20 is a type of burner with tires as the fueland with a supply of air or oxygen like most any conventional burner,similar to what was described previously. In this embodiment, however,the container 20 would include at the bottom of the container a cleanoutopening for the metal residue. The tires in the container would beignited so that they would decompose into combustible vapors and gasses.Just enough oxygen would be supplied to maintain this state, thuscausing the tires to melt with the metal within the tires beginning toseparate therefrom. The combustibles would then flow to an area wherethey would be mixed with additional oxygen and blown or drawn into thearea where the heat was needed, such as a boiler or similar furnacefirebox. The metal would separate and settle to the bottom of thecontainer, as the melting point of the metal is higher than the meltingpoint of the rubber. Not enough heat would be generated to melt thesteel, just enough to separate the rubber materials from the steel. Inthis manner, the tires are broken down into combustible fluids that canbe moved into an area where they can be burned as fuel while leaving themetal behind for disposal.

In a more complex device for a much larger application, the burnercombustion would take place in a sloped rotary kiln, with thecombustible products as vapors or gases leaving the upper opening andthe metal waste discharging from the lower opening. The metal would bethen be recyclable as steel scrap.

In either case the metal scrap would be condensed and much more easilyhandled then the wire that is currently generated when processing scraptires. The cost would be much lower also when compared to conventionalscrap tire processing with de-wiring and separation of the rubber fromthe metal. The inherent problems in the use of tires as fuel and theseparation of the metal from the rubber are thus solved and the rubbercan be burned for additional fuel while also allowing the scrap metalfrom the tires to be reused in a desired format.

It is contemplated that the inventive concepts herein described may bevariously otherwise embodied and it is intended that the appended claimsbe construed to include alternative embodiments of the invention exceptinsofar as limited by the prior art. For example, various types offurnaces may be used with the present invention, and the scrap tires maybe of different types. Furthermore, different types of metals may bemelted in the furnace besides the scrap metal, including such metals asiron pellets, direct reduced iron (“DRI”), pig iron and iron carbide,which are often scrap substitutes.

There has therefore been shown and described a process for melting scrapmetal which accomplishes the stated objectives.

1. A process for melting steel scrap for producing steel in a furnacecomprising: providing a container adjacent to the furnace, wherein thecontainer is a rotary kiln; providing a quantity of scrap rubber intothe container; providing at least one burner to ignite the quantity ofscrap rubber in the container to convert the quantity of scrap rubberinto combustible liquids and gasses; separating metal residue from theliquids and gasses; and feeding the converted scrap rubber into thefurnace to add additional heat for melting the metal.